In a medium bowl stir together flour, baking soda, salt, and pumpkin pie spice.
With an electric stand or hand mixer on medium-high speed, in a large bowl cream butter and sugar until light and fluffy. Beat in eggs one at a time, mixing well after each addition. Scrape down sides of bowl.
With the mixer on medium speed, add vanilla, pumpkin puree, and vegetable oil until completely combined.
Add the dry ingredients in thirds, alternating with the milk, beginning and ending with the flour mixture.
Divide batter evenly into 3 greased pans. Bake 35-40 minutes. Cake is done when a cake tester or toothpick inserted in the middle comes out clean. Let the cakes cool 15 minutes and then remove from the pans and cool completely on a wire rack. Wrap individually in plastic wrap and freeze to make frosting the cakes easier.
Buttercream Frosting
With a stand mixer on medium speed, cream the butter until light and fluffy, about 3 minutes.
Add 4 cups powdered sugar, milk, and maple extract and beat on low speed until it's smooth. Gradually mix in 2 more cups of powdered sugar on low speed. Beat until light and fluffy, adding up to 1 more cup of powdered sugar to reach desired consistency. If desired, add a few drops of gel food coloring to all or some of the frosting (In separate bowls if you want to make an ombré effect.)
Remove cakes from freezer and remove plastic wrap. With a long, serrated knife, slice tops off to make flat. Place bottom layer cut side down on serving plate or on a cardboard cake round (minimum of 9") on cake turntable. Spread with a layer of frosting. Place second layer cut side down onto bottom layer. Spread with frosting. Place the third cake layer top side up on the second layer. Frost top and sides with frosting.
Store in airtight container at room temperature up to two days.